1. Field of the Invention
The present invention is in the field of amplifiers, and relates particularly to amplifiers employed for driving loads such as speakers which are reactive and are also subjected to mechanical distortion influences such as inertia.
2. Description of the Prior Art
For about the past forty years, and still under the current state of the art, audio amplifiers have employed what is commonly referred to as "voltage feedback" to improve their frequency response and reduce distortion. Such voltage feedback system are sometimes referred to as "constant voltage" systems, since for a fixed amplifier input voltage the output voltage remains substantially constant over a broad frequency range or bandwidth. Thus, current audio amplifiers are capable of providing an output voltage for driving a speaker which quite accurately follows the amplifier input program voltage, as to both wave shape or form and phase.
However, the conventional speaker used as a load for the amplifier has both electrical characteristics and mechanical characteristics which prevent it from coming even close to following the voltage output but instead cause the speaker to depart considerably from the program applied by the amplifier in amplitude, wave form and phase, and the general result is that the acoustic response of the speaker is considerably different than the flat voltage response of the amplifier.
The conventional speaker is an inductively reactive load, and this electrical characteristic of the load creates a number of different effects which adversely affect speaker response to the program. One such effect of the inductive reactance is that it causes the load impedance to vary with frequency, the impedance becoming higher, and power to the speaker consequently lower, at higher frequencies.
The inductive reactance of the speaker load also causes the load current to lag in phase from the program, and this phase lag varies with frequency similar to the impedance, becoming much greater at higher frequencies. The current phase lag adversly affects speaker response in several different ways. Thus, it reduces the amount of power to the speaker, and this reduction is greater at higher frequencies. Much of the power that is not applied to the load because of the phase lag is, instead, dissipated as heat in the amplifier output, and this in turn requires larger, more expensive output transistors. The phase lag, in general, distorts the wave form of the program and tends to mask program detail.
Conventional amplifier voltage feedback systems are not able to provide correction for either the impedance variation with frequency or the phase lag, and the industry has simply learned to live with these adverse effects of the inductive reactance of the speaker load. Record companies have employed various types of compensations, particularly in an endeavor to improve the frequency response, but such compensations are only partially helpful and introduce their own distortions into the program.
The principal mechanical factors which adversely affect speaker performance are inertial lag and overshoot resulting from the mass of the speaker, and various resonances, particularly the open air cone resonance of the speaker, but also speaker cabinet resonance and even room resonance.
The adverse effects of inertia, and particularly of intertial overshoot, on the performance of audio systems have apparently not heretofore been fully understood, and certainly have not been treated with the seriousness that they deserve; nevertheless, applicant has determined by comparison of the acoustic response of conventional systems with that of the system of the present invention where the effects of inertia are substantially completely removed, that inertia is one of the most important factors which deteriorates speaker response in conventional systems.
Inertial lag in the conventional speaker system causes the speaker to trail behind sharply rising wave fronts in the program and thus not adequately respond to high frequency overtones and transients in the program. On the other hand, inertial overshoot causes several problems, including a similar failure to respond to sharply falling wave fronts, masking of high frequency overtones and transients, and the generation of spurious current signals in the load which result in corresponding spurious sounds that not only interfere with applied portions of the program, but overhang into open portions of the program and thereby completely cover up the important sound effects of room or hall ambience.
Inertial overshoot and the variations of phase with frequency have adverse effects on complex wave forms that are somewhat similar. Thus, in the case of inertial overshoot the speaker tends to go right on past high frequency overtones and transients, covering them up and replacing them with spurious signals; while in the case of phase lag variations with frequency, the high frequency overtones and transients have a much greater phase lag than the fundamental or basic program pulses, and are thereby in effect left behind the program to become spurious signals. Accordingly, in relatively complex wave forms such as those found in music, the combination of inertial overshoot and phase lag variations with frequency results in a portion of the speaker acoustic output being noise content that is unrelated to the music program. These interferences of phase lag and overshoot with the high frequency overtones, together with the increased speaker impedance at high frequencies, resulted in serious formant distortion in music and voice programs.
Many of the aforesaid problems in audio systems are also present in various other electrically driven loads having electrical reactance, or tending to have inertial lag or overshoot, or having other mechanical factors tending to alter movement of the load other than that prescribed by applied program. Thus, cutting heads for sound records have essentially the same inductive reactance and inertial problems as a speaker. Capacitive loads, such as capacity speakers, have reactance characteristics which cannot be matched by conventional amplifiers. Direct writing galvanometers, such as pen-writing recorders, and even optical galvanometers such as those employed for motion picture sound, have reactance and inertia problems similar to those of a speaker. In the driving of machinery, where the load is highly reactive and the machinery involves considerable inertia, some of the problems discussed above for speakers are magnified. Thus, the dissipation of large amounts of power in the amplifier output because of phase lag is expensive and difficult to cope with in this type of equipment. Problems similar to those of speakers are also found in vehicle directional control equipment such as automatic pilot controls for aircraft and ships, and remote control equipment for driving steering elements of aircraft, ships, and large land vehicles.
The very poor impedance/frequency response of a single speaker is improved in most audio systems by the use of multiple-speaker arrangements with crossover networks, and such systems can become quite expensive, employing a large number of speakers. However, such systems do not solve the more serious problems resulting from the inductive reactance of speakers of phase lag and phase lag variations with frequency; and such systems make no attempt to solve the major mechanical problems of inertial drag and overshoot. Crossover networks, although improving impedance/frequency response, nevertheless introduce further problems, including phasing problems and sharp impedance rises proximate crossover points.
Various speaker cabinet designs, some of them quite elaborate and expensive, are also employed in an endeavor to reduce speaker mechanical problems and thereby improve response. While some cabinet designs do reduce open air cone resonance at low frequencies, they introduce further problems such as speaker cabinet resonances and undesired damping, and they do not cure the major mechanical defects of inertial drag and overshoot.
The approach that the art is taking today in attempting to reduce speaker inertial overshoot is to raise the "damping factor" to as high a value as possible. The term "damping factor" as used in the art is the ratio of rated speaker impedance to amplifier output impedance. Damping factors of as high as 800 or more are claimed in the industry. However, in rating the "damping factor", the art completely ignores the fact that, insofar as actual damping is concerned, the speaker is, in effect, looking into its own resistance in series with the amplifier output in the damping circuit loop, so that the best actual damping factor obtainable would be on the order of about 1.3. Accordingly, this passive circuit approach does not do much to help counteract speaker inertial overshoot.